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Loco PositioningAn Open Source Local Positioning System
Arnaud TaffanelBitcraze AB
FOSDEM 2017
Crazyflie 2.0
● Open source flying development platform● Designed to be expandable in both software and
hardware○ 168MHz Cortex-M4 CPU with FPU (stm32f405)○ Deck expansion port
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Our autonomous flight attempts
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The decawave DW1000
● Radio chip available of the shelf● Standard based: IEEE802.15.4a UWB PHY● 500MHz bandwidth with 5 channels from 3 to 7GHz● Radio that can range: potential to be used as a base for a
Local Positioning System
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What is a local positioning system ?
● Similar to GPS but local (ie. indoor)● Provides absolute position
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Why a local positioning system ?
● Indoor navigation● Assets tracking● Bitcraze focuses on indoor positioning and navigation for
robotics
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Existing systems: Optical
● Motion capture systems○ State of the art for flying robotic and swarm research
○ Very precise but expensive● Optical flow mounted on platform
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Existing systems: Radio-based
● Received signal strength (ex. Bluetooth low energy tag)○ Coarse accuracy
● Angle detection○ Angle of arrival○ Azimut from the transmitting antenna (ex. VOR system used
for airplanes)● Time of flight
○ Requires wide bandwidth to be resilient to multipath
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Ultra Wideband (UWB) radio: What is it ?
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Ultra Wideband radio: Multipath
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Ultra Wideband: Packet format and timing
● Very long preamble● SFD (start frame delimiter) is the timestamped instant● Packets can carry up to 127 Bytes of data
○ 1024 with a Decawave proprietary extension● Packets contains source and destination address
○ IEEE802 MAC header (MAC addresses)
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Ultra Wideband: Preamble detection
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Ultra Wideband radio: Timestamping
● Precise timestamping of packets at transmission and reception○ 64GHz timer, 1.5ps timer tick -> ~5mm○ Decawave DW1000 specifies +/-100mm distance
measurement accuracy● Robust to multipath● Not so robust to non-line-of-sight (NLOS)● NLOS induces an offset measurement
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UWB-based LPS architecture
● Anchors are part of the infrastructure: UWB radios placed at known location
● Tag is what we want to locate: mobile UWB radio14
Two Way Ranging (TWR): 2 Packets
● Basically ping:
● Anchor and tag have different clocks: very big error if anchor response time is not close to 0
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Two Way Ranging (TWR): 3 Packets
● One exchange added to cancel clock drift error
● Now there is information left in the anchor
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Two Way Ranging (TWR): 4 Packets
● Last packet transfers timestamps to the tag● Active distance measurement
○ The tag controls the distance measurement rate○ Bi-directional communication
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Two Way Ranging (TWR): Positioning
● Tag at intersection of○ 2D: circles○ 3D: spheres
● Noise properties scale well with distance
● Good performance in and near the Anchor convex hull
● Requires active distance measurements: does not scale with number of Tags
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Time Difference of Arrival (TDoA)
● If packets were sent at the same time○ Difference between receive time is the difference of time flight○ Can be used to calculate relative distance to Anchor 1
compared to Anchor 2
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Time Difference of Arrival (TDoA)
● Sending many packets at the same time is not possible so:○ We assign one time slot to each anchor○ At the reception the time slot time is subtracted from TDoA○ Two packets from the same anchor can be used to synchronize
the local clock to the remote clock● How to synchronize transmit times between anchors?
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Time Difference of Arrival (TDoA)
● Packets are broadcast● The same packet are used for measuring time of flight
between anchor (TWR!)○ If we have time of flight between two anchors we can
synchronize their clock
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Time Difference of Arrival: Positioning
● Tag at intersection of○ 2D: parabola○ 3D: paraboloid
● Tag needs to be in anchor convex hull for good precision
● Scales very well: Tags just listen
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Loco Positioning System (LPS)
● Based on of-the-shelf UWB radio: DWM1000● Open source local positioning system for robotic
○ Currently with a focus on Crazyflie 2.0● Useful for robotics and more generally anything that
needs real time absolute positioning23
LPS Architecture: Nodes
● STM32F072 Cortex-M0 MCU● DWM1000 UWB module● Firmware using FreeRTOS● Open source DW1000 driver● Can be used as Anchor, Tag
or UWB sniffer○ This is why it is called Node
and not Anchor● Upgradable and configurable
via USB○ Radio (OTA) update and
configuration in development24
LPS Architecture: Crazyflie 2.0 deck
● Based on DWM1000 UWB module
● Ranging, positioning and control implemented in Crazyflie 2.0 firmware
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LPS Architecture: Firmware
● TWR stable, TDoA experimental● More than just ranging required for autonomous robotics
○ Positioning using sensor fusion (Kalman)○ Position and trajectory control
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LPS Architecture: Software
● ROS Support● Support by Crazyflie lib and client in development● System configuration and management tools in
development
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Project status and future
● Currently used by universities and industry● Tech artists very interested by an open flying swarm for
shows● Lots of software planed or in development
○ Blender choreography authoring plugin○ Swarm management software○ Automatic anchor position measurement
● Small Tag with IMU and LPS planned
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Our next autonomous flight attempts?
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Demo!
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Non-linear Quaternion controller
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